The invention relates to disconnect switches for use with the start or auxiliary winding of a capacitor-start capacitor-run single phase AC induction motor.
The invention relates to continuing development efforts and improvements over the circuitry shown in Bossi et al U.S. Pat. No. 4,782,278, assigned to the assignee of the present invention, and incorporated herein by reference. The invention also arose from continuing development efforts relating to U.S. Pat. Nos. 4,604,563, 4,622,506, 4,658,195, 4,687,982, 4,670,697, 4,745,347, 4,751,449, 4,751,450, 4,782,278 and 4,786,850, also assigned to the assignee of the present invention.
A capacitor-start capacitor-run single phase AC induction motor is subject to a phenomenon known as "first cycle current spiking", and to another phenomenon known as "cycle to cycle current spiking". The first cycle current spiking is experienced in both mechanical and solid state disconnect switches.
The present invention addresses and solves the first cycle current spiking problem, and also addresses and solves the cycle to cycle current spiking problem.
As is known in the prior art, a single phase AC induction motor has a main winding and an auxiliary winding both connectable to an AC power source. A start capacitor provides a phase shifted field for starting torque. A semiconductor power switch automatically connects and disconnects the start capacitor to and from the AC source in starting and running modes, respectively. The gate current for the semiconductor power switch is derived through a resistor, such as resistor 72 in the above noted Bossi et al patent, connected to a node common to the start capacitor and a main terminal of the power switch. This circuitry performs admirably on capacitor start or split phase motors. However, when applied to capacitor-start capacitor-run motors, a gate circuit of this type is subject to the above noted first cycle current spiking and cycle to cycle current spiking.
One solution to current spiking known in the prior art is to provide a choke for the run capacitor, such as a large inductance or coil connected in series with the run capacitor or an extended winding. The choke is undesirable because it is massive and expensive. Furthermore, the choke does not solve the first cycle current spiking problem. The choke does reduce the current spike, but not enough to be satisfactory. Another disadvantage of the choke is that it requires the insertion of an element in series with the run capacitor, which is costly from a manufacturing standpoint. Another disadvantage of the choke is that it is in the circuit continuously.
The present invention provides a solution which is particularly simple and effective, and eliminates the need for a separate series connected choke.
The present invention solves the first cycle current spiking problem by controlling when, in relation to the AC cycle, the power switch is allowed to turn on, rather than just allowing the power switch to turn on at random during any point in the AC cycle upon initiation of the starting mode. Instead, the start switch may be turned on only within a known or predictable range of voltage differential between the run and start capacitors.
The present invention solves the cycle to cycle current spiking problem by controlling phasing of gate current to the start switch to be in phase with the voltage across the run capacitor and out of phase with the current through the series connected start switch and start capacitor.